So-called square-wave operation is in widespread use in electronic control gear for gas discharge lamps. In this case, a direct current whose polarity is reversed at regular intervals is applied to the lamp. The polarity reversal is necessary in order to avoid electrophoresis effects and in order to subject each electrode of the lamp to an even load. Direct current operation is also in widespread use in gas discharge lamps which are designed specially for this purpose. For particular applications, for example in the projection operating mode, however, rapid changes in current are also required in DC lamps. Very rapid changes in current are also required in projection applications with semiconductor light sources in order to ensure a constant color temperature.
The direct current is generally provided by a pulsed power supply, also referred to below as DC/DC voltage converter. This usually involves known topologies such as buck converters, boost converters, buck-boost converters, Cúk converters or SEPIC converters. The voltage which is supplied to the DC/DC voltage converter is usually higher than the voltage at the light source, for which reason a buck converter is preferably used. The power which the DC/DC voltage converter can provide to a load is generally set by the duty factor or the switching frequency of electronic switches which are contained in the DC/DC voltage converter. For this purpose, the DC/DC voltage converter has a setting input, at which a setpoint value is fed in. If the DC/DC voltage converter is a buck converter, for example, a pulse-width modulator (PWM) is usually used, which converts the setpoint value into a drive signal for the electronic switch contained in the buck converter.
If rapid changes in current now occur in the setpoint value, for example as a result of the polarity reversal of the lamp current or as a result of changes in color in projector systems, the DC/DC voltage converter cannot follow these rapid changes in current. The change in current takes place more slowly and an overshoot response occurs, i.e. the actual value of the current goes beyond the setpoint value and only approaches the setpoint value again after a certain time. As a result, the light source draws more power, which is reflected in an increased, often undesirable light emission. In the case of the projection systems, for example, the color representation can be incorrect as a result of this effect. Owing to the temporarily increased current, undesirable noise emitted by the circuit to the surrounding environment thereof owing to magnetostriction and other mechanisms is also increased.
EP 1 326 483 A1 therefore attempts to avoid the overshoot response as a result of the polarity reversal of the lamp current by virtue of the setpoint value of the current being reduced shortly before the polarity reversal and the desired setpoint value being set again shortly after the polarity reversal. The setpoint variable is therefore decreased in synchronism with the polarity reversal and therefore in synchronism with the switchover signal by a decrease value with a time characteristic. Although this reduces the overshoot response during polarity reversal of the lamp current, i.e. during commutation of the lamp, a contribution cannot always be made to rapid changes in the lamp current which result in a rapid change in the lamp power.